The present invention applies to an aircraft which, for navigation, uses positional information supplied by a conventional satellite positioning system, for example of the GPS (the English-language abbreviation for “Global Positioning System” is commonly used), GALILEO or Glonass type. It is known that a satellite positioning system such as this uses a constellation of satellites capable of transmitting information to a moving object which information can be used by this object particularly in order to determine its position and speed.
Although not exclusively, the present invention applies more particularly to a transport airplane, particularly a military one, which is able, using an appropriate on-board management system, to carry out a phase of flight close to the ground, which corresponds:                either to a low level flight of the LLF (the English-language acronym for “Low Level Flight” is commonly used) type;        or an autonomous approach, of AA (the English-language acronym for “Autonomous Approach” is commonly used) type.        
It is known that:                a low level flight is a flight during which the aircraft flies at a low level, generally following the map of the earth, that is to say following the contour of the terrain overflown, particularly in order to avoid being spotted (especially by ground radars). For such a low level flight, the pilot may input into the onboard system which manages this flight a setpoint value for a set clearance height of the SCH (the English-language acronym for “Set Clearance Height” is commonly used) type which is, for example, around 500 feet (about 150 meters). The low level flight path that is to be followed by the aircraft during this phase of flight is then calculated by said management system, in the usual way, at least at this set clearance height SCH which is defined above the contour of the terrain overflown; and        an autonomous approach represents the approach to a landing zone, which is performed autonomously, that is to say without using facilities external to the aircraft and onboard means, particularly radar means liable to be detected from the outside for example, from ground surveillance stations. During an autonomous approach such as this, the aircraft pilot inputs into the onboard system which manages the approach a setpoint value for a decision height of the DH (the English-language acronym for “Decision Height” is widely used), which is for example around 200 feet (about 60 meters). This decision height DH represents the minimum height at which a decision with regard to landing has to be taken during this autonomous approach. In particular, an aborted approach procedure has to be begun if the visual contact needed to continue the approach with a view to landing has not been made at this decision height.        
Particularly for obvious safety reasons, the aforementioned two phases (low level flight, autonomous approach) of flight close to the ground require very careful monitoring of the position of the aircraft, particularly with regard to its vertical component, which position is supplied by a satellite positioning system.
In order to achieve such monitoring, usually:                an alarm threshold is determined and associated with the setpoint value (set clearance height SCH or decision height DH) selected by the pilot;        repeatedly, a current error value is received which corresponds to an estimated current error in the vertical position (supplied by said satellite positioning system), and this current error value is compared against said alarm threshold; and        if the current error value passes above the alarm threshold, an alarm is emitted.        
The aforementioned alarm threshold therefore allows the aircraft to be protected at the level required by the corresponding phase of flight, and in the event that an estimate of the error value exceeds this alarm threshold, an alarm is triggered to indicate to the pilot that his navigation is degraded and that he needs to interrupt the mission he is flying.
The above approach has a major disadvantage namely that it is not possible to anticipate the triggering of the alarm and therefore to modify the mission (should that prove necessary in order to be able to accomplish it). Specifically, as soon as an alarm is emitted, the pilot has immediately to interrupt his mission and increase the altitude of the aircraft, for obvious safety reasons, because the position supplied by the positioning system is no longer reliable enough for the conditions under which the mission is to be carried out.
It is an object of the present invention to remedy the aforementioned disadvantages. The invention relates to a method for monitoring the ability to navigate an aircraft during a phase of flight close to the ground (low level flight or autonomous approach), said aircraft, particularly a transport airplane, using, for its navigation, positional information supplied by a satellite positioning system, said method particularly making it possible to anticipate the triggering of an alarm and, if appropriate, to modify a mission parameter so that the aircraft will be able to complete the assigned mission.